The present invention relates to an air flow meter comprising a heating resistor provided in an air passage. The flow of air is measured, and a constant temperature control circuit keeps the temperature of the heating resistor at a predetermined value by controlling a current supplied to the heating resistor. The invention particularly relates to a heating resistor type air flow measurement apparatus suitable for the measurement of the intake air flow in an automobile engine.
In recent years, a method of controlling the amount of fuel supplied as a function of intake air flow has generally been used in engines (internal combustion engines) for automobiles. In such a control method, an air flow meter is widely used. A heating resistor type air flow measurement apparatus is one such air flow meter, and today, is in mainstream use.
An output signal of the heating resistor type air flow measurement apparatus is supplied, for example, to an ECU (engine control unit) of an automobile. The output signal is supplied, for example, to control the amount of fuel supplied to the engine. While the output signal is supplied, it is necessary to perform predetermined adjustment of an air flow signal or calibration.
By calibration is meant scale matching of instruments and adjustment so that the zero point and the spun (sensitivity) fall within predetermined specification ranges.
In the prior art, the flow-detection characteristics of each heating resistor type air flow measurement apparatus is checked and adjusted so as to fall within a range of specifications before product-shipment.
A resistance trimming method of adjusting a value of the resistance of a resistor is widely used today as such an adjusting method. By using the resistance trimming method, the zero point and the spun are adjusted so that the measurement output may fall within the range of specifications.
An alternative to the resistance trimming method is described in U.S. Pat. No. 4,669,052. This alternative is described as "an electronic trimming" or a multi-points correction method which uses digital data.
In the multi-points correction method, correction is made by using the digital data instead of the resistance value of a resistor. A map in which the predetermined data are stored is retrieved, an air flow is corrected by obtaining an air flow value and a flow error, and a signal indicative of the correct flow is output.
The characteristics of the flow correction data QaMap (q,a,b) used are checked for every apparatus, after product completion, and the data required to correct the error are calculated and maintained. The correction processing by the flow correction data QaMap (q,a,b) corresponds to the adjustment processing in resistance trimming. The trimmed and calibrated flow value is obtained as an after-correction flow value QaRef.
In the prior art, the flow correction data QaMap (q,a,b) for trimming are stored in the primary memory means, for example, before product shipment. The flow correction data QaMap (q,a,b) are read from the primary memory means when the operation of the apparatus is started up; for example, these data are read when turning the power switch on. The flow correction data QaMaP (q,a,b) read are stored in the predetermined memory set on a program area or the secondary memory means to be used as a Q map.
As the primary memory means, for example, EEPROM (Electrically Erasable/Programmable Read Only Memory) is used. This is a kind of nonvolatile memory which makes it possible to read/write data at any time.
In the above-mentioned prior art, the reliability of the flow correction data stored in the secondary memory means is not taken into consideration. There is, therefore, a problem in that an abnormal measurement flow value is output, as it is, even when the flow correction data become abnormal.
If the secondary memory is set on the program area, then the probability that the table data (Q map) stored in the secondary memory means to correct the flow will become abnormal is low. However, there is a possibility that the data set before product shipment will change, because the original flow correction data stored in a memory, such as the EEPROM or the primary memory means, can possibly be read/written after product installation.
Therefore, a write-protect for the data is provided, except for the adjusting step for the product. In the above-mentioned prior art, however, the data are read from the EEPROM, for example, when turning the power on. Consequently, there is fear that the data are written erroneously. As a result, the flow correction data actually used in the correction process may become abnormal.
When the data are read erroneously, the data for use in the correction of the flow error become abnormal, and thus suitable correction can not be made. As a result, the output value of a sensor (data after correction) does not fall within the range determined by the specifications, and the control of an engine becomes abnormal.